In recent years, fluorinated polymers have been used for various applications utilizing their characteristics, since they are polymer materials excellent in the heat resistance, solvent resistance, chemical resistance, etc.
As a method for producing fluorinated polymers, a solution polymerization method, a suspension polymerization method or an elusion polymerization method is known. As a polymerization medium for the solution polymerization method or the suspension polymerization method, an inert solvent such as a chlorofluorocarbon is usually employed from the viewpoint of the polymerization speed or for a reason that it presents a copolymer of high molecular weight. Specific examples of such a chlorofluorocarbon include trichlorofluoromethane, dichlorodifluoromethane, trichlorotrifluoromethane and dichlorotetrafluoroethane. From the viewpoint of the handling efficiency, trichlorotrifluoroethane is mainly used.
Whereas, in recent years, destruction of the ozone layer has been taken up internationally as a problem of environmental destruction of global scale, and as a responsible substance, the chlorofluorocarbon has been pointed out, and there is worldwide trend for total abolition of its use. Therefore, it is now necessary to stop using the chlorofluorocarbon which has been used at the time of producing fluorinated polymer.
As a substitute for such a chlorofluorocarbon, a hydrofluorocarbon having fluorine atoms, has been proposed, as it has a small ozone-destructing coefficient. However, a substance having a C--H bond has been known to show a chain transfer property to a fluoroolefin, and it has been believed that it is difficult to use a hydrochlorofluorocarbon containing hydrogen atoms, as a polymerization medium for the production of a fluoroolefin-type polymer of a high molecular weight. As other substitutes for the polymerization medium, t-butanol (Japanese Examined Patent Publication No. 24073/1977), etc. are known, but it is thereby necessary to conduct polymerization under a high pressure in order to obtain a polymer having a sufficiently high molecular weight.
Further, when these substitutes are used as polymerization media, a commonly employed chain transfer agent, such as methanol, may sometimes be hardly compatible with such polymerization media and, in such a case, is not useful as a chain transfer agent. Further, a perfluorocarbon or hydrofluorocarbon has poor affinity with the polymer, whereby handling efficiency tends to be poor in a step after polymerization (such as granulation or drying).
Under these circumstances, the present invention has been accomplished for the purposes of providing a chain transfer agent whereby a fluorinated polymer excellent in the heat resistance, solvent resistance and chemical resistance can efficiently be produced when a perfluorocarbon or hydrofluorocarbon is used as a polymerization medium, and providing a method for efficiently producing a fluorinated polymer excellent in the heat resistance, solvent resistance and chemical resistance, whereby the polymerization rate is high and the molecular weight of the fluorinated polymer can sufficiently be increased, without using a chlorofluorocarbon having a large ozone destruction coefficient.